An organic EL device is a self-emitting device that is based on a principle according to which, with an electric field applied, a luminescent material emits light by recombination energy caused by holes injected from an anode and electrons injected from a cathode.
Organic EL devices formed from organic materials have been vigorously studied since a report on a low voltage-driven organic EL device formed by laminating layers was made by C. W. Tang et al. of Eastman Kodak Company.
Moreover, a phosphorescent organic EL device in which a phosphorescent material is used as a luminescent material has been proposed. The phosphorescent organic EL device can achieve a high luminous efficiency by using a singlet state and a triplet state of excited states of a phosphorescent material. The reason is presumed as follows. When holes and electrons are recombined in the emitting layer, it is presumed that singlet excitons and triplet excitons are produced at a rate of 1:3 due to difference in spin multiplicity. Accordingly, luminous efficiency of the device using a phosphorescent material can reach three to four times as much as that of the device using only a fluorescent material.
Thus, an organic EL device provided with two or more emitting layers for enhancing luminous efficiency has been proposed (see, for instance, Patent Literature 1 and non-Patent Literature 1).
Patent Literature 1 and non-Patent Literature 1 disclose an organic EL device provided with a first emitting layer and a second emitting layer that are continuous with each other.
In the organic EL device of Patent Literature 1, a host material of the second emitting layer is a monoazine derivative having a higher electron transporting performance than a host material of the first emitting layer. Luminescent materials of the first and second emitting layers are the same ortho-metalated complex.
In contrast, in the organic EL device of non-Patent Literature 1, luminescent materials of the first and second emitting layers are different ortho-metalated complexes (see device A in FIG. 1).